As a constant velocity universal joint used in a power transmission device of an automobile, various industrial machines, or the like, there is a tripod type constant velocity universal joint using a tripod member as an inner joint member. The tripod type constant velocity universal joint includes, for example, as illustrated in FIG. 15, an outer joint member 1, a tripod member 2 serving as the inner joint member, and torque transmission members (roller members) 3.
The outer joint member 1 includes a cup-like mouth portion 4 opened at one end thereof. In an inner peripheral surface of the mouth portion 4, there are formed three track grooves 6 extending in an axial direction of the outer joint member. In side walls facing to each other in a circumferential direction of each of track grooves 6, there are formed roller guide surfaces (roller sliding surfaces) 7a, 7a. 
The tripod member 2 includes a boss 8 and leg shafts 9. The boss 8 is provided with spline hole or serration hole 11 which is coupled to a shaft (not shown) to be allowed to transmit torque. The leg shafts 9 each protrude in a radial direction from positions trisecting the boss 8 in the circumferential direction.
Further, each of the torque transmission members 3 includes: a roller 12 including a ring-like body including a radially-outer surface 13 formed into a convex spherical surface; and a ring 15 fitted into the roller 12 through intermediation of a plurality of rollers 16. That is, the roller 12 and the ring 15 are unitized through intermediation of the plurality of rollers 16. Those members and the like constitute a roller assembly. In this case, the roller 12 can be called an outer roller, and the ring 15 can be called an inner roller.
By the way, the outer joint member 1 includes: the mouth portion 4 including the track grooves 6; and a stem portion (not shown) provided to protrude from a bottom wall of the mouth portion 4. Further, a radially-inner surface of the mouth portion 4 exhibits a shape of a three-petal corolla of a flower, in which smaller radially-inner portions 7b and larger radially-inner portions 7c are alternately connected together through intermediation of the roller guide surfaces 7a in the circumferential direction. That is, in the outer joint member 1, the roller guide surfaces 7a facing to each other in the circumferential direction and the track grooves 6 including the larger radially-inner portions 7c each provided between both of the roller guide surfaces 7a, 7a are formed at three positions in an inner periphery of the outer joint member.
The tripod type constant velocity universal joint illustrated in FIG. 15 uses so-called double-roller type roller members each including the inner roller and the outer roller. However, as illustrated in FIG. 16 and FIG. 17, the tripod type constant velocity universal joint may use so-called single-roller type roller members each including a roller fitted onto a cylindrical outer peripheral surface of the leg shaft through a plurality of needle rollers in such a manner that the roller is rotatable. In this case, each of the leg shafts 9 includes a cylindrical outer peripheral surface 74, and an annular ring groove 76 formed in vicinity of an end of each of the leg shafts. A roller 80 is fitted onto an outer periphery of each of the leg shaft 9 through a plurality of needle rollers 82 in such a manner that the roller 80 is rotatable. The cylindrical outer peripheral surface 74 of each of the leg shafts 9 provides an inner raceway surface for the plurality of needle rollers 82. The roller 80 includes a cylindrical inner peripheral surface to provide an outer raceway surface for the plurality of needle rollers 82.
The plurality of needle rollers 82 are held in contact with an outer washer 84 on outer end surfaces of the plurality of needle rollers, and with an inner washer 88 on end surfaces opposite to the outer end surfaces of the plurality of needle rollers, when viewed from a radial direction of the leg shaft 9. The outer washer 84 is regulated from moving in the axial direction by a circlip 86 mounted into the ring groove 76. As a result, the plurality of needle rollers 82 are also regulated from moving in the axial direction.
Generally, the roller guide surfaces 7a of the outer joint member 1 are subjected to heat hardening treatment. As the heat hardening treatment, there is induction quenching. The induction quenching is mainly divided into a one-shot quenching method of performing heat treatment at a fixed position (Patent Literature 1), and a mobile quenching method in which the coil is moved relative to the outer joint member (Patent Literature 2).
In the one-shot quenching, there is used, as illustrated in FIG. 10 and FIG. 11, a high-frequency heating apparatus including a high-frequency induction heating coil 17. In this case, the high-frequency induction heating coil 17 includes heating portions 17a, 17a, 17a respectively fitted into the three track grooves 6 of the outer joint member 1. Therefore, it is possible to both heat and rapidly cool the roller guide surfaces 7a over an axial entire region of the joint at one time. Further, in Patent Literature 1 described above, in order to prevent hardened layers from being formed in inner surfaces of the larger radially-inner portions 7c, non-conductive ferrite cores are fitted in an induction coil. Therefore, by using the above-mentioned high-frequency induction heating apparatus, there are formed hardened layers S only in surface layers of the roller guide surfaces 7a as illustrated in FIG. 11.
Further, in the mobile quenching method, another high-frequency induction heating apparatus illustrated in FIG. 12 to FIG. 14 is used. The high-frequency induction heating apparatus includes: a coil 18 wound to have a three-leaf clover shape; and a cooling jacket 19 attached to the coil 18. The coil 18 includes heating portions 18a, 18a, 18a each having substantially triangle shape and being provided at about 120° pitches in a circumferential direction of the coil. Each of the heating portions 18a includes: curved portions 20a, 20a respectively opposed to the roller guide surfaces 7a; and a straight portion 20b opposed to the larger radially-inner portion 7c. Note that, the cooling jacket 19 has a three-leaf clover shape similarly to the coil 18.
In the mobile quenching method, the coil 18 and the cooling jacket 19 are moved along an axial direction in the mouth portion 4 of the outer joint member 1. Along with the above-mentioned movement, heated surfaces (roller guide surfaces 7a and larger radially-inner portions 7c) are heated. Following the above-mentioned heating, the heated surfaces are rapidly cooled with cooling water injected through the cooling jacket 19. Thus, the roller guide surfaces 7a and the larger radially-inner portions 7c are subjected to the heat hardening treatment. In this manner, the hardened layers S are formed.
Citation List
Patent Literature
[PTL 1] Japanese Examined Patent Publication No. Sho 61-34481 [PTL 2] Japanese Examined Utility Model Publication No. Hei 03-26335